The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Vehicles use powertrain systems to generate tractive torque, including hybrid powertrain systems that transfer torque through a transmission to an output member. Known hybrid powertrain systems for motor vehicles include internal combustion engines and torque machines that transfer tractive torque to one or more drive wheel(s), including transferring torque through a transmission device to an output member. The output member can be coupled to a driveline for transferring tractive torque thereto. One known hybrid powertrain system includes electric machines that are configured to operate as torque motors and electric power generators that exchange electric power with an electric energy storage device. Another known hybrid powertrain system includes hydraulic machines that are configured to operate as torque motors and hydraulic power generators that exchange hydraulic power with a hydraulic power accumulator.
Known powertrain control schemes include generating a low level of tractive torque subsequent to an operator command for vehicle braking. A low level of tractive torque causes the vehicle to maintain a slow, preferably constant vehicle speed, referred to as creep, when the vehicle is on a level surface with operator-applied braking and with no operator-applied braking and no operator input to an accelerator pedal. A low level of tractive torque, i.e., a creep torque can cause a vehicle to begin a low speed forward motion when a vehicle operator releases a brake pedal without applying the accelerator pedal when the vehicle is on a flat surface. Furthermore, a low level of tractive torque holds the vehicle and prevents rollback when a vehicle operator releases a brake pedal while the vehicle is on an inclined surface. This is intended to be responsive to an operator expectation that the vehicle remain stationary and not roll backwards. The low level of tractive torque is preferably generated by the torque machine(s) of the hybrid powertrain, consuming some level of stored power and generating heat in the torque machine(s). Known hybrid powertrain systems include generating a creep torque that decreases at a fixed rate relative to operator brake pedal input.